The phrase "are difficult to mark" is correct and usable in written English.
It can be used when discussing tasks, assignments, or items that present challenges in evaluation or assessment.
Example: "The essays submitted by the students are difficult to mark due to their varying quality and unclear arguments."
Alternatives: "are hard to evaluate" or "are challenging to assess".
It's difficult to mark the exact moment when a radical advocate chooses to cross the line separating civil disobedience from violent crime.
"Carroll scored a great goal and was difficult to mark.
Obviously it could be difficult to mark up items for sale, but the team are considering a few options.
Among blind people, voting presents an obvious obstacle: It's difficult to mark a ballot if you can't see it.
Because the Bourbon fleur-de-lis remained the factory mark, it is difficult to distinguish the later Italian from the early Spanish ware, though inevitably the standards of Capodimonte were not long maintained.
In this study, male germ cells irradiated with over 0.5 Gy of carbon ions are difficult to repair completely marked by the sperm count.
The management of marking workload within a short timeframe is immense, particularly with teachers trying to mark everything themselves, which can cause marking backlogs that are difficult to be cleared (Sutton, 2010), adding more burden and demanding more time and greater commitment.
The waxy gray seed clusters should be collected as soon as the leaves wither, but they are difficult to spot, so Mr. Cullina suggests marking the plants while in bloom.
However, many species are difficult to sight, and cannot be easily marked or recaptured.
These modifications are associated with a plethora of important biological functions, yet the mechanistic contributions of individual ADP-ribosylation marks are difficult to dissect due to a dearth of (bio chemical tools to study this diverse class of PTMs.
